Apparatus for indication of small capacity variations



Mamh 1954 K. JOHANSSON ET AL ,673,340

APPARATUS FOR INDICATION OF SMALL c PAcITY VARIATIONS Filed May 7, 1949 IOTPUUZZO U MHZ/Qua! 2/0 Ih llllllllll IMIII,

Patented Mar. 23, 1954 APPARATUS FOR INDICATION oFsMALL CAPACITY VARIATIONS Karl Lennart J ohansson, GrondaL'and Per Harry Elias Claesson, Jakobsbei'g, Sweden ApplicationMay '1, 1949, 'Serial No. 92,068 Claims priority, application Sweden May 15,1948

4 Claims.

The present invention relates to apparatuses which indicate when an object penetrates into a certain area. The apparatus comprises an electronic tube high frequency generator which is connected to an antenna device in the said area. Penetration into the area by the object changes the capacity between the antenna and earth. The indicating device in such apparatuses usually consists of a relay which is connected in such a way that an alteration in the current through its winding takes place when the circuit is affected, for example, when itsfrequency or oscillation amplitude is changed.

Animportant field for the use of such apparatuses is the keepin of guard, especially burglar alarms. Up to the present, however, no such apparatus'has been produ'ced'for practical use. This is a consequence of lackof stability. In alarm and guard devices, where the apparatus must work day afterday without attention inan absolutely reliable manner, it is an indispensable condition that it shall be stable i. e. in the mains, independent of variations in temperature or in the power mains.

In the present invention the said antenna device, the high-frequency generator and a-sensi- 'tive device for detecting capacity variations between the said antenna device and-earth which forms a capacity between the antenna device and earth are isolated with an isolating material having a mainly constant dielectric constant for normal temperature variations.

Preferably'the invention comprises an oscilatting circuit, included in the high-frequency'electronic tube generator, and connected in the customary manner between the grid and another electrode, preferably the cathode of an electronic tube. Characteristics of the invention are that the connecting elements to the grid side of the circuit are so arranged that their resulting capacity is mainly constant for normal variations of temperature and that the impedance between the earth side of the circuit andthe earth is small compared to the impedance between the said earth side and the power mains. The'manner in which both sides of the oscillating circuit must be connected up in order to attain'full stability will be'explained further with reference to the annexed figure which shows an example of construction according to'the invention.

Theelectronic generator consists of an e1ectronic tube E andan'oscillating circuit L1C'in a three-point coupling ofknown type. To the grid ciilrcuitbfthe tube,viatlie' condenser 01' and'ithe allel-a's a fe'le'r. In the anode circuit of the; tube,

a relay R, adjusted for its limiting position, is connected as an indicating device. The apparatus is fed from the A. C. mains V via a transformer T together witharectifier L and a filter condenser 04.

The diagram shown of the apparatus, which is known in principle, functions as-fo'lldws:

If L2=L3 (the tapping point is in the middle of the coil L1) ,the tension between the tubes E-grid and cathode is small when 01:03 and -R1=the radiation resistance of the antenna, so that oscillations are not produced. If C1 'is smaller than C3, no oscillations are produced either, because the oscillations which mig'htbe supposed to arise between the grid and the cathode are out of phase. Whenon the-contrary C1 is'greater than C3, self-oscillations are produced which feed the antenna. By reason of heavydamping in the oscillating circuit, chieflydependent on the resistance R1 the degree of oscillationof the tube'can be regulated at will, for example toh'alf the 'maximum amplitude. This adjustment is made by means of the condenser ci. The said elements coil L1, condensers C1 and C3 and the tube E form thus a high-frequency generator. When the tube is oscillating, it operates as a grid rectifying detector, i. 'e. the grid receives a negative'potential in relation to the cathode. The value of this potential is dependent on the degree of oscillation "attained by the tube, which oscillationin turn is dependent on the value of thecapacit'y C3. The

negative potential of the grid causes the'anode current to fall. The said "high frequencygen'erator which includes the said rectifying device forms a sensitive device, i. e., sensitive for variations of the capacity 03.

As'will be further sho'wn,'according to themvention, the connecting "elements v connected to the circuit L1G, as well asthe circuit itself, must be arranged in a special mannerif the apparatus is to be stable. ;In this connection, the grid side of the circuit willjbe dealt with first.

A good apparatus will give indication of a variation of 0.1 mmf. or less in Ca,"preferably0.01 mmf. It should be clear from the diagram that ifthe capacity between the grid of thetube and the cathode or the grid 'andthe anode varies, this will cause mainly the'same" change in'current in the anode circuit of the tubeas if 'av'ariation in capacity was produced betweenthe antennaand the earth. In normal variations of temperature (:20" 0.), sucha capacityvariation is normal in electronic "tubes. The electronic tube, "there* fore must be speciaHy'ctinstructed "for "the pres ent purpose. A first measure to be taken is to construct the tubes with the least possible gridanode and grid-cathode capacities. This is achieved, amongst other things, by taking out the grid terminal direct through the wall of the tube with the shortest possible leads not running parallel to the leads from other electrodes. Further, the electrodes in the tube must be fixed with material possessing a small heat-expansion coefficient, for example steatite or something similar. Special attention must also be paid to the connection between the grid and the antenna and to the antenna itself, so that a stable capacity is produced in the grid side of the circuit, not variable with changes of temperature and humidity. This naturally also applies to the oscillating circuit itself. A compensating device may alternatively be resorted to. With changes of temperature it must have such changes in capacity that the changes in capacity of the other elements connected to the grid circuit are compensated.

As for the earth side of the circuit, it is necessary for the impedance Z1 of the element, for example the coupling condenser to earth which forms the capacity G3 with the antenna, to have a small impedance. Since the distance between the earth connection J2 of the apparatus and the earth J1 of the antenna can usually be made small, a suitably small and stable impedance can be obtained on this connection also. On the other hand, the impedance between the earth side of the circuit and the mains must be high for high-frequencies. This is due to the fact that the impedance of the mains to the earth J3 may vary considerably, and thereby the resulting influence which the earth of the mains exercises over Z1 via Z3 may vary, which in its turn affects the circuit L1G and causes faulty indication. Z3 consists in this case partly of the impedance of the apparatus to the mains and partly of the mains impedance to earth. The manner in which the impedance of the mains can be varied is clear from the figure. The mains leads are supposed to have a certain length, for example overhead leads, so that the impedance Z: can be arranged to come in series with the leads. If thereafter a load B3 with relatively large capacity C2 is connected to the earth, the impedance is naturally decreased between the mains side of the apparatus and the earth, which implies that Z1 is more heavily shunted. When the apparatus is connected to the mains via a transformer with electrically separate primary and secondary windings, or even with an earthed screen between windings, a fairly high impedance is produced between the two windings. In the case of a so-called universal-current or D. C. apparatuses a suitable impedance coil is connected between the apparatus and the mains. Ihis may be of the so-called anti-disturbance type. In order to obtain a sufficiently high impedance, several such anti-disturbance filters, with their component condensers if any, must be connected up.

The above willnow be further illustrated by an example of a universal current apparatus. Let (13:50 mmf. The sensitivity of the apparatus, i. e. the variation of 03 under the influence of the relay=0.01 mmf. or 2 10- of C3. Z1 is a condenser of 5000 mmf. The example shows that the same variation in the circuit as under the influence of the relay (0.01 mmf.) can be obtained if Z1 varies by about 0.75 mmf. and that a yariatiQn-of the impedance Z2 between, for

example, a, and 50,000 ohms may also cause the said variations in Z1. From this it follows that a particularly effective filter must be connected between the mains side of the apparatus and the mains and that Z1 must be chosen of low value. The usual value of Z1 (0:5,000 mmf.) is generally too small and must be increased to at least 10 times as much.

By connecting the transformer between the apparatus and the mains, there is obtained, in the known manner, an electrical separation of the apparatus from the mains. For this reason, the circuit L1C can be connected to the earth lead J2 through Z1 which, in its turn, by means of a lead of low impedance shown as a dotted line, is brought forward to the antenna earth J1, whereby such a low-resistance connection is obtained between the earth points J1 and J2 that the transformer Ts capacity G5, which should be able to be maintained about mmf. or even lower, constitutes a suflicient uncoupling between the mains and the apparatus. In all circumstances, the impedance between the earth side of the circuit and the mains should be one hundred times greater than the impedance between the same side of the circuit and the earth.

We claim:

1. Apparatus with means of mains feeding for the indication of the penetration of an object into a certain area comprising an antenna device in the said area, at least one body for insulating the said antenna device from earth, an electronic tube coupled partly asa high-frequency generator with an oscillating circuit coupled to earth by means of a coupling element, and partly as an amplifier with a condenser and a resistance in parallel connected on one side to the oscillating circuit and on the other side to the antenna device, the said condenser fixed so that the oscillation amplitude strongly depends on the size of the capacity between the antenna device and earth, the said coupling element having a small damping effect on high-frequency current compared with the damping efiect to the earth of the means of mains feeding, and the said body having a substantially constant dielectric constant for normal variations of temperature and humidity.

2. Apparatus with means of mains feeding for the indication of the penetration of an object into a certain area comprising an antenna device in the said area, an electronic tube with cathode, grid and anode, said tube coupled partly as a high-frequency generator with an oscillating circuit and partly as an amplifier with a condenser and a resistance in parallel connected on one side to the oscillating circuit and on the other side to the antenna device and the control grid in the tube, the said oscillating circuit also coupled to the cathode and to earth, the said condenser fixed so that the oscillation am: plitude strongly depends on the size of the capacity between the antenna means and earth, the said oscillating circuit being coupled to earth by means of a coupling element with small damping eifect on high frequency current compared with the damping effect on the means of mains feeding, and insulating body for the control grid in the tube forming a capacity between the said grid and anode and cathode with less than 0.1 mmf. capacity variation for 20 degrees Celsius temperature variation in the tube.

3. Apparatus with means of mains feeding for the indication of the penetration of an. object into a certain area comprising an antenna device in the said area, at least one body for insulating the antenna device to earth, an electronic tube coupled partly as a high-frequency generator with an oscillating circuit and partly as an amplifier with cathode, anode and control grid, the said cscillating circuit comprising an inductance with a tapping point, a condenser and a resistance in parallel connected on one side to the grid and on the other side to one end of the inductance, the tapping point of the inductance being connected to the cathode and the other end of the inductance to earth, the antenna device also being connected to the grid, the said condenser fixed so that the oscillation amplitude strongly depends on the size of the capacity between the antenna device and earth, the said inductance being coupled to earth by means of a coupling element with small damping effect on high frequency current compared with the damping effect on the means of feeding and insulating material forming a capacity between the antenna device and earth having a mainly constant dielectric constant for normal temperature variation.

4. Apparatus with means of mains feeding for the indication of the penetration of an object into a certain area comprising an antenna device in the said area, an electronic tube coupled partly as a high-frequency generator with an oscillating circuit and partly as an amplifier with cathode, anode and control grid, the said oscillating circuit comprising an inductance with a tappmg point, a condenser and a resistance in parallel connected on one side to the grid and on the other side to on end of the inductance, the tap-ping point of the inductance being connected to the cathode and the other end of the inductance to earth, the antenna device also being connected to the grid, the said condenser fixed so that the oscillation amplitude depends on the siz of the capacity between the antenna means and earth, the said inductance being coupled to earth by means of a coupling element with small damping effect on high frequency current compared With the damping effect on the means of feeding and insulating material for the control grid in the tube forming a capacity between the said grid and anode and cathode with less than 0.1 mmf. capacity variation for 20 degrees Celsius temperature variation in the tube.

KARL LENNART JOHANSSON.

PER HARRY ELIAS CLAESSON.

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